(a) Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device having a hard mask coupled to a discharge plug and, more particularly, to a technique for forming a cylindrical hole receiving therein a via-plug or a stacked capacitor by using a hard mask.
(b) Description of the Related Art
A DRAM device has an increasing number of memory cells by employing a reduced design rule. The reduced design rule inevitably reduces the occupied area of each memory cell, and thus requires a reduced occupied area for a stacked capacitor used in the memory cell without reducing the capacitance thereof. A cylindrical capacitor is generally used as the stacked capacitor for storing data in a memory cell, to achieve a reduced occupied area and yet a larger capacitance. The cylindrical capacitor is generally received in a cylindrical hole, and includes a bottom electrode formed on the bottom and sidewall of the cylindrical hole, a capacitor insulation film formed thereon, and a top electrode filling the cylindrical hole to oppose the bottom electrode with an intervention of the capacitor insulation film. In general, a larger depth for the cylindrical hole receiving therein the stacked capacitor provides a larger capacitance for the stacked capacitor.
However, the larger depth for the cylindrical hole may involve a bowing structure of the cylindrical hole such as shown in FIG. 6. More specifically, the increase of the depth for the cylindrical hole 104 causes the bowing structure wherein a larger side etch of the cylindrical hole 104 generates an increased width of the cylindrical hole 104 at a specific depthwise position 105 near the top of the cylindrical hole. In the same drawing, 101, 102 and 103 denote a silicon substrate, a thick insulation film receiving therein the cylindrical hole 104, and a hard mask used for etching for forming the cylindrical hole 104, respectively. The bowing structure causes a malfunction of a smaller thickness of the insulation film between the bottom electrodes of adjacent capacitors, and may eventually cause a short-circuit failure between bottom the electrodes.
The reason for occurring of the bowing structure is such that a larger depth of the cylindrical hole involves a smaller etch rate in the vicinity of the bottom of the cylindrical hole, and increases the time length of etching for the cylindrical hole to thereby increase the side etch at the specific depthwise position. Patent Publication JP-11-354499A, for example, describes a technique for suppressing the bowing structure, wherein an etching gas including CHF3 and CO used in a dry etching process allows a carbon-rich film to be deposited on the sidewall of the cylindrical hole, the carbon-rich film acting as a protective film for the sidewall to suppress the side etch of the cylindrical hole.
Recent giga-bit-class DRAM devices having a reduced design rule of 0.1 μm include a cylindrical hole having a depth of 2.5 to 3.0 μm, a diameter of 0.16 μm, and thus an aspect ratio of 15 to 19. In this range of the aspect ratio, even the technique described in the above publication cannot effectively suppress occurring of the bowing structure even if the etching gas is optimized for the dry etching.
The present inventor presumed that the bowing structure for the case of a high aspect ratio resulted from the profile of the amount of a polymer, which is generated during the dry etching and deposited onto the sidewall of the cylindrical hole. In this presumption, it was noted that the thick insulation film made of SiO2 and receiving therein the cylindrical hole generates the polymer as the product of reaction between the SiO2 film and the fluorocarbon (CxFy) used in the etching gas, that the polymer having a high deposition rate has a tendency of attachment onto the sidewall in the vicinity of top of the cylindrical hole, resulting in that the specific depthwise position of the cylindrical hole was attached with a less amount of polymer and thus rapidly etched by the side etch. However, a sample including a cylindrical hole having a depth of 2.5 μm and exposed to gas plasma using a CF-containing gas exhibited that the above presumption was not correct, because a sufficient amount of polymer was observed as attached onto the area of the cylindrical hole from the vicinity of the top to the center thereof.